Underworld

The Underworld is the afterlife for souls in Greek mythology, located beneath the planet of Earth. It was invisible to the living, as it was the kingdom of the dead, unlike Mount Olympus, the shining kingdom of the gods.

History
The Underworld was the realm of Hades, the God of the Dead, who was given control over those who died and passed to the afterlife. The underworld was where souls went (in the forms of their former selves) and were judged, and they would be given a pleasant or dark afterlife. Those who were favored by the gods would go to Elysium, while those whom the gods were hostile to went to Tartarus or remained in the underworld, a realm of lava and fire. The underworld's inhabitants were the shades, and the dead were prevented from escaping by Hades' three-headed dog, the Cerberus, which also prevented mortals from entering the underworld.